Darlene just shotgun blasting anyone that got on her nerves mad me laugh.
With Del is was actually the right decision.
Had Del lived, he would have forced Marty to keep pushing and keep building new local alliances until Darlene and Jacob could have been replaced, then there would be no reason to leave them alive.
One of the best arguments against Del having the desire to kill Marty in the pilot episode is that Del was actually in worse trouble for the theft than anyone else. It was long standing/long running scam against the Cartel. And Del let it happen.
Del's character was built to constantly test people. Test their limits, hence the parable about the elderly cashier who stole money from the register to pay for her son's medicine. Only Marty said to fire her and never look back. The rest rationalized the behavior. Why kill Marty too when you can give him another week or so to see what you can squeeze out of him.
As soon as Marty's old partner Bruce saw Del in the pilot episode, he should have already been prepared to run and already been armed to the teeth and should have immediately started shooting. Bruce's major mistake was getting involved in the first place. If he chose that path, then he should have had a fallback plan/run package prepared. Then he needed to face the reality that one day he'd have to shoot his way out.
So Darlene was correct, or maybe she was accidentally right. Disrespect in that line of work is the same thing as announcing they are going to kill you very soon.
The "correct" decision with Del would have been to gut shoot him, then put him in a van full of fertilizer and take him to a big city and in a very public and crowded place. Use Del's phone to send in threats. Then trigger it.
Now the Navarro Cartel is a priority for enacting terrorism on US soil. They would be too busy fighting US Special Forces and the DEA to have time for small time gangsters in the Ozarks. Make no mistake, under that scenario, the DEA, FBI, DHS, US SOCOM and all the rest would start executing every last Cartel member they could find.
Frank Sr was stupid ( now you know where his son's temper comes from), he uttered a threat that was pointless. If you are going to kill her, then kill her.
This is where Javi the nephew is somewhat capable. Javi knows he's no genius and no long game tactician. Easier to just start killing people and keep killing them until the board is clear.